True....you have a resonant circuit. Not a resonant antenna.
----- Original Message -----
From: "Jim Brown K9YC" <k9yc@audiosystemsgroup.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Monday, July 07, 2008 6:12 PM
Subject: Re: [TenTec] OT SWR VS Power Loss
> On Mon, 7 Jul 2008 14:52:10 -0700, Alfred Lorona wrote:
>
> >Zero reactance in a wire antenna feedpoint is a necessary but not
sufficient
> >condition for it to be truly resonant. Citing the Antenna Fundamentals,
chap
> >2, page 2-1 of my 16th addition of The ARRL Antenna Book, a resonant wire
is
> >defined as 'The shortest length of wire that resonates at a given
frequency
> >is one just long enough to permit an electrical charge to travel from one
> >end to the other and back in the time of one RF cycle.'
>
>
> >Clearly, a short wire with a finite pure resistive radiation impedance is
> >too short physically for the required condition to exist as stated in the
> >Handbook definition..
>
> >On the other hand, elsewhere in the book, a zero reactance 'resonant'
> >condition does not mention the required physical length of the wire. If
this
> >is all that is required, ANY wire can be called resonant and there is
> >nothing to distinguish such wires from a halfwave wire. Somehow this
doesn't
> >sit right with me. What do you think? Do you think that this discrepency
> >begs for a minor clarifying rewrite?
>
> The text is fine -- you missed an important part of the sentence -- "in
the
> time of one RF cycle." That establishes the length of the antenna as long
> enough to be resonant. It will only be "very short" if the frequency is
very
> high! When the antenna is resonant by the definition of the text it will
be
> resistive.
>
> It is also possible to resonate a non-resonant antenna by tuning out its
> reactance. Here's a simple example. A quarter wave vertical typically has
less
> than 30 ohms radiation resistance at resonance, but if we make it longer,
we
> can find a length where the impedance at the feedpoint is 50 +jxxx ohms.
We can
> then add a capacitor in series to tune out the inductance (+jxxx), and the
> antenna now looks like 50 ohms and matches the coax. Now, we have a
resonant
> circuit that includes the antenna and the capacitor.
>
> 73,
>
> Jim K9YC
>
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|